The present invention relates to a novel ultimate analyzer for analyzing elements of an object to be analyzed based on an output signal of a scattered electron beam and a plurality of output signals of an electron beam energy dispersed after passing through an object to be analyzed, and a scanning transmission electron microscope having the ultimate analyzer and an ultimate analysis method using the scanning transmission electron microscope.
With progressing of miniaturizing and downsizing of semiconductor devices and magnetic head elements, the structure of these elements has a structure that thin films of several nm (nanometers) are laminated in an area of a sub-micrometer order. Since the characteristics of the semiconductor elements and the magnetic head elements strongly depend on the structure, the element distribution and the crystal structure in such a micro-area, it is important to analyze them in the micro-area.
As the means for observing a micro-area, there are a scanning electron microscope (SEM), a transmission electron microscope (TEM) and a scanning transmission electron microscope (STEM). Only the TEM and the STEM have a spatial resolution of a nanometer level. The TEM is an apparatus in which an electron beam is irradiated onto a sample, and the transmitted electron beam is magnified using a lens. On the other hand, the STEM is an apparatus in which an electron beam is focused onto a micro-area, and a two-dimensional image is obtained by measuring intensities of the transmitted electron beam while the electron beam is being scanned on the sample.
As the means for observing a two-dimensional distribution of elements on a plane of a sample, there are an energy dispersive X-ray spectroscopy (EDX) and an electron energy loss spectroscopy (EELS) using the TEM or the STEM. For example, in a case of analyzing a metal film, Cr, Mn, Fe, Co, Ni and Cu can be identified using the energy dispersive X-ray spectroscopy, and two-dimensional distributions of the above metals can be obtained.
On the other hand, by using the electron energy loss spectroscopy, silicon, oxygen and nitrogen can be identified, and two-dimensional distributions of silicon, silicon oxide and silicon nitride can be observed. The electron energy loss spectroscopy is a method of analyzing lost energy for exciting inner-shell electrons of elements composing a sample when the electron beam transmitting through the sample. The electron that lost energy due to the excitation of the inner-shell of the element to be analyzed is called as core-loss electron. The ultimate analysis can be performed because the lost energy is specific to an element, and a two-dimensional distribution of the elements can be observed by performing energy analysis in each position in the plane of the sample. These spectroscopy are widely used by combining the STEM and a parallel detection type electron beam energy loss spectrometer.
The parallel detection type electron beam energy loss spectrometer comprises a magnetic-prism spectrometer; quadrupole electromagnetic lenses and hexapole electromagnetic lenses arranged at the front of and at the rear of the magnetic-prism spectrometer; and a parallel detector arranged after the magnetic-prism spectrometer. The quadrupole electromagnetic lenses are used for adjusting focus of the electron energy loss spectra and for magnifying the electron energy loss spectra. The hexapole electromagnetic lens is used for reducing aberration of the electron energy loss spectra projected on the detector. The electron energy loss spectra magnified by the quadrupole electromagnetic lens is projected on the parallel detector to measure a wide range of the electron energy loss spectra.
The prior art in regard to the structure of the parallel detection type electron energy loss spectrometer is disclosed in, for example, U.S. Pat. No. 4,743,756, Japanese Patent Application Laid-Open No. 7-21966, Japanese Patent Application Laid-Open No. 7-21967, and Japanese Patent Application Laid-Open No. 7-29544. An electron energy analyzer is disclosed in Japanese Patent Application Laid-Open No. 57-80649.
In a conventional apparatus combining the parallel detection type electron energy loss spectrometer and the STEM, a user performs (1) specifying a measured position, (2) specifying an element, (3) measuring an energy intensity distribution of the electron beam using the electron beam detection part, (4) correcting background of the detection part and correcting the gain of the detection part, (5) specifying a background region of the spectrum, (6) specifying a background fitting function such as the power-low model (I=A×E−r; A and r are coefficients, and E is energy), (7) specifying an integration region of the signal intensity, (8) displaying the signal intensity of the specified element in the measured position on the image display unit, and (9) performing the operation of the item (1). Since it is necessary to perform the repetitive operation described above for all the measuring points, it takes a long time to obtain a two-dimensional image, and accordingly it is difficult to obtain an element distribution in real time. Further, it can be considered to obtain the two-dimensional image by the method that after measuring the electron energy loss spectra for all the measured points, the user specifies the operations of (2) to (7). In this method, the volume of measured data becomes very large, and further, the element distribution image can not be obtained in real time.
In addition to the above, in the case where the element distribution image can not be obtained in real time, there are following problems:
(A) In a case where analysis of an interface between thin films, the analysis region (the interface between thin films) can not be identified by using a TEM/STEM image when measuring the electron energy loss spectra. Accordingly, whether or not the region to be measured is included in the analyzed region cannot be judged until the element distribution image is obtained after analyzing the electron energy loss spectra.
(B) The conventional analyzer is not suitable for the work such as the inspection to measure many samples because it requires the measurement of the electron energy loss spectra and the many complicated and complex operations for each measured point, and also it requires a long time for the measurement and the analysis.
(C) In a case of identifying an oxide film or a deposited element formed in an interface between dissimilar metals, it cannot be identified by observing only a distribution image of the single element which metal between the dissimilar metals is oxidized, or it is difficult to be identified by observing the element distribution image whether the elements exist on the interface between the dissimilar metals or are distributed inside one of the metals.
Further, in an analyzer which detects an element to be analyzed by dividing an intensity of a first electron beam in an energy range containing a core-loss peak among the electron energy loss spectra of the element to be measured by an intensity of a second electron beam in an energy range higher than the core-loss peak, which is called as a jump-ratio method, there is the following problem depending on the sample to be analyzed.
When light elements such as oxygen, nitrogen and the like are observed in a case where a heavy metal element exists in the sample to be measured, a portion of the heavy metal element is sometimes displayed with brightness similar to brightness of the distribution image of the light elements. In that case, since the contrast difference between a metal portion and an oxide or nitride portion becomes small, it becomes difficult to judge correctly existence of oxide or nitride.
As described above, the analyzer combining the electron energy loss spectrometer and the STEM is difficult to observe an element distribution image having high contrast in real time and to determine the distribution of the element with high accuracy.
On the other hand, as a means for preventing degradation of an image due to brightness variation of an electron source in STEM image observation using a scanning transmission electron microscope, Japanese Patent Application Laid-Open No. 2000-21346 discloses a scanning transmission electron microscope in which one of an output signal from a detecting means for detecting transmission electrons transmitted through a sample and an output signal from a detecting means for detecting scattered electrons scattered by the sample is divided by the other. However, this patent discloses a means for improving image quality of a STEM image, but not a means for analyzing elements of the sample.
The literature by K. Kaji, et al., “Light Element Mapping Method with Scanning Transmission Electron Microscope”, The Japanese Society of Electron Microscopy, May 17, 2000: p307, describes a method for obtaining an oxygen distribution image, in which gate electrodes are dark and oxidized films thereon are bright.
According to the description, using a transmission electron scanning microscope of a field-emission type and an ultimate analyzing observation apparatus, the method obtains such image by logical dividing operation applied to an oxygen distribution image acquired by 2-window method using a Z-contrast image as the divisor.
However, this method is still not enough to respond to the demand for more contrasted screen-displaying of gate electrodes and oxidized films thereon.